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UNITED STATES f
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% a:.G.m f JAN.I 8 1978 s i, Mr. Steve Gadler I]
Board Member 3
Minnesota Pollution Control Agency h
1935 West County Roa.d 82 3
Roseville, Minnesota 55113
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Dear Mr. Gadler:
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This is in response to your letter of December 19, 1977, to former s
Chairman Marcus Rowden concerning the offgas explosion at the Millstone Nucicar Power Station, Unit No.1, on December 13, 1977. We have provided answers to each of your questions in the enclosure to this letter.
b We gather from the thrust of some of your questions that you believe the consequences of these events are greater than they actually are.
It q
should be recognized that the hazard of explosion from the hydrogen a
mixtures was considered during the design of the offgas system.
The I
design is intended to withstand any explosion that occurs within the offgas system. The utilities' designs and the NRC staff revicws have been successful in that mechanical damage and radioactive releases have been within design limits, with minor exception, for the twenty-five -
known explosions which have occurred within offgas systems of boiling water reactors. There also have been four offgas explosions which have occurred outside the offgas systems, including the latest explosion at Millstone Unit No.1.
Details on these accidents are provided in the enclosure.
Because the offgas systems are designed to withstand the effects of f
offgas explosions, automatic reactor scrams have not been required to 4
assure that limiting conditions of operation, as specified in the Technical Specifications of the NRC license, are satisfied. When necessary, controlled reactor shutdowns employing load reduction and/or manual scrans have been used to bring the reactor to an operational mode which would permit utility investigation of the cause and effect of l
the offgas system explosion.
'i As part of the NRC staff effort to maintain releases as low as reasonably
,1 achievable (Appendix I, 10 CFR 50), augmented offgas systems are being or have been added to all large boiling water reactors. As part of the new systems, hydrogen recombiners will be utilized and will greatly reduce the potential for collection of the explosive hydrogen mixtures.
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- r. Steve Gadler.
E'ecats of this :ature are routinely i:isele.:cd to tha oublic in several utys. Licensce event repochs are placed in public document rec ~s near titility sitz.'.n.ditient.llv. l.ha utili ties,w /c.c t :a I.T.C R,nf omi ONico staf.s -.y nctify state and icc.sl <;c./creeant officitic.~ press a
reirascs are mda by utilitics and/or the !!T:C if :nv radicactivity is rrit. :T! to pi rsitc-areas or if person >;rl inf tn v cr.d/cr rech:nical d: cge results.
4, U: trust that this inforestica, clcng with-the encicsure, answers your
' A q'te:tions.
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Sincerely.
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ANSWER T012/19/77-LETTER FROM MR. S. GADLER
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MINNES0TA POLLUTION CONTROL AGENCY Question: "I understand that there were two ' hydrogen' explosions at Millstone #1 in Connecticut on December 14, 1977. There are conflicting stories about what happened, consequently, I am writing to you.
"I am interested in getting answers to the following questions, with your assistance:
l.
What happened at Millstone #1 on Decembpr 14, 1977?"
Answer:
The stack and associated equipment at Millstone iluclear Power Station, Unit No.1, experienced two hydrogen gas explosions on December 13. One was at about 9:30 a.m., and did only modest damage in the offgas system which is designed to withstand such anticipated explosions. A second, at about 1:00 p.m., blew the door off a room under the stack. After the first explosion, controlled shutdown was initiated; after the second, the rdactor was shutdown immediately because of the ground release of radioactive effluents.
Question: "a. Were there any injuries? If so, how many, how serious?
Current status?."-
Answer:
One worker, passing by outside at the time of the second explosion, was thrown to the ground and received some con-tamination with radioactive material. He was decontaminated i
and treated for lacerations and possible concussion at a nearby medical facility. He was released after treatment for the concussion, i
No significant exposures to radiation or radioactive material occurred.
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Question: "b.
Were there any deaths? If so how many?"
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l Answer:
There were no deaths.
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l Question: "c.
How much radioactive material, in curies, was released l
into the air, surface and groundwater and which radio-p nuclides were involved?"
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Answer:
The first offgas explosion at Millstone Nuclear Power Station, f
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Unit No.1, did not result in any significant release because I
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the explosion occurred with'in the offgas system. The maximum calculated release of radioactive material for the second explosion, considering the volume of offgas, the offgas 4
activity at the power level at the time of the explosion, and assuming an instantaneous ground level release of the entire offgas system, was 54 curies. The release consisted of the normal mixture of radioactive noble gases and fo#ine which other-wise would have been discharged from the top of'the stack.
The calculation did not account for the strong chimney effect and normal dilution of the stack.
New thermolumin:: scent dosimeters at the ' downwind site boundary recorded less than one millirem. Ground level contamination at the stack base had decayed to background levels the next day.
l Question: "d. Has such an accident happened at Millstone Units No. l d
or 2 in the past? If so, when and how serious was it?"
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Answer:
There was no such accident at Millstone Nuclear Power Station, Units No.1 or 2, in the past.
Question: -"2. Have such ' hydrogen' explosions occurred at the
- Nor.thern States Power nuclear plant at Monticello, Minnesota?"
Answer:
There have been offgas hydrogen explosions at Menticello.
l Question: "a. If so how many times?
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- b. Dates of such accidents" i
Answer:
Four explosions occurred within* the offgas system at Monticello j
nuclear power plant in November 1971, May 1974, June 1974 and j
July 1974.
Question: "c. Mcw much radioactive material, in curies, was ' discharged i
into the-air, surface and ground water and which radio-nuclides were involved?"
Answer:
On two occasions, there was no release of radioactivity because either the reactor had been shut down prior to the 3
explosion or the offgas system had been isolated due to a high pressure condition.
In the other two events, 3 curies total activity (halogens, noble gases, tritium and other i
fission products) were released.
The total releare is based on the release rates at the time of the explosion.
Question: "d. Were there any injuries.
If so how serious and what was the final disposition of the injuries?
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Were there any deaths? If so, how many?"
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. Answer:
There were no deaths or injuries resulting from these offgas explosions.-
Question "e.
Why were the people of Minnesota not informed of any of those accidents if in fact,they occurred?"
Answer:
The licensco issued an advisory notice to the newspapers for the first event and'the NRC issued press releases for the second and third events when there were releases of radio-activi ty. A press release was not issued on the fourth event because the system had been isolated and no release occurred.
The Minnesota-Pollution Control Agency was notified of the last three events, two by copics of Licensee Event Reports to NRC and the last event by licensec letter on April 4,1975.
Minnesota officials were not informed of the first event because there were no injuries, no releases, and no mechanical' damage.
Question: " Finally, I was not aware that accidents such as the one at Millstone No.1 involving release of radioactive materials into the air, surface and groundwater and injury to worhers were occurring.
Recently, I had been told that such acci-l.
dents are very common in boiling water nuclear power reactors 4
(BWR's).
If this is true, I want to know the magnitude of this probicm and what the Nuclear Regulatory Commission is I
doing about it.
I would appreciate the.following information:
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i 1.
Do such " hydrogen" explosions occur at other nuclear power plants in the United States? If so:
)
a.
Where have such accidents occurred?
j b.
Dates of thesp accidents?"
Answer:
The nuclear power stations and the dates of the explosions are shown below:
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Plant Date Plant Date Dresden 1 8/71 Dresden 3 11/74 Monticello 11/71 Dresden 2 S/75 Lacrosse 6/72 Cooper 11/75 Quad Cities 2 3/73 Cooper 1/76 -
Dresden 2 3/73 Brunswick 2 1/76 Vermont Yankee 6/73 Cooper 4/76 Vermont Yankee 8/73 Dresden 3
.4/76 Vermont Yankee 9/3/73 Dresden 2-6/76 a
Vermont Yankee 9/9/73 Oyster Creek 8/77 Dresden 3 2/74 Dresden 1 9/77 Quad Cities 1 3/74 Browns Ferry 7/77*
Monticello S/74 Vermont Yankee 12/10/77 Monticello 6/74
. Millstone 1 12/13/77 Monticello 7/74.. e
- No explosion reported; Quad Cities 1 10/74 elevated temperature observed in offgas charcoal filters.
Question: "c.
Amount of radioactive material, in curies, released.
into the air, surface and groundwater and the radio-nuclides involved?"
Answer:
Following an offgas explosion, much of the radioactivity was released up the stack and was within technical specification 1
limits established by the NRC.
In only one case (Dresden Unit 3, November 1974), the effluent release limit (for halogens) was exceeded by 40 percent for a two hour period.
In three cases where the explosion occurred external to the offgas system there were near ground level releases.
Site boundary measurements did not disclose any levels exceeding 10 CFR Part 20 limits.
f The radionuclides released included noble gases, halogens, tritium and other fission products which are routinely released by boiling water reactors.
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Question: "d.
Were any workers or other staff injured.
If so how many and current status? Were any killed? If so, how many? Where?"
Answer:
In one explosion at Cooper (November 1975) which took place external to the offgas system, one plant employee received i
blast burns. Two workers received minor injuries at Dresden Unit 2 (March 1973).
No deaths occurred.
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. Questibn: "e. Were any of these plants " scrammed:7 If so, which ones and what was the reason? When did the scram occur?"
Answer:
In most of the events, operating conditions did not reach any of the reactor trip settings.
In some instances, operators
. recognize that conditions might reach levels which would require reactor shutdown, and an orderly shutdown process, if which may have included a manual scram, was employed.
Manual scrams were initiated for the following offgas explosions events:
q a.
Dresden Unit 3 (February 1974) - The reactor went through a load reduction and manual scram to reduce L
the effluent _ release and to permit repair of the off-gas system.
b.
Dresden Unit 3 (November 1974) - A manual scram was used to shut down the reactor because of loss of condenser vacuum.
c.
Cooper Nuclear Station (January 1976) - The reactor was manually ' scrammed to tenninate a ground-level release. The manual scram was initiated about 2 minutes after the explosion. The calculated release rate was less than the maximum permissible concentra-tion in 10 CFR Part 20.
d.
Bruns'vick Nuclear Station (January 1976) - The explosion which occurred in a room at the base of the stack damaged the access hatch.
Power reduction was ini-tiated about 10 minutes after the explosion,and the reactor was scrammed about 20 minutes after the explosion to terminate a ground release.
Question: "2.
Is the Nuclear Regulatory Commission engaged in monitoring for radionuclides released from existing nucicar power plants? If so, is this data avail-able.
If not, why is NRC not monitoring suc.h discharges for release of radionuclides?
"a.
Does NRC do any monitoring during these ' hydrogen' explosions:
If not, why not?"
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. s Answer:
Th'e NRC staff does not routinely conduct environmental monitoring activities around NRC licensed nuclear facilities, sb:
The licensee has technical specification or license conditions Nl imposed by the NRC which requires him to conduct environmental monitoring programs. The results of these programs are r
'I available and can be found in the public document room.
a Independent environmental measurements are performed for the NRC under contracts with many states in which reactors are
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located. The State of Minnesota does not have such a contract.
Question: "3.
What action, if any,' has NRC taken to date' to stop the discharge of radionuclides into the environment? What plans for the future?"
Answer:
The NRC limits the discharge of radioactive materials from nuclear power plants by: (1) evaluating the initial design of the power plants and associated systems, (2) limiting the quantities of radioactive material that can be discharged from the facilities through technical specification in the operating license, and (3) requiring the addition of augmented offgas systems to reduce offsite discharge.
Plans for the future include continued evaluation of additional j
cleanup techniques which are not developed or economically feasible at this time.
Question: "4.
What action has NRC taken to date to stop or prevent the occurrence of these ' hydrogen' explosions at existing nuclear power plants?"
Answer:
To meet the design objectives of Appendix I,10 CFR 50, utilities are often required to add augmented offgas systems. Within the augmented systems design, hydrogen recombiners will minimize the potential for the explosive mixtures. In general, the NRC staff investigates the causes and inquiries into corrective actions taken by utilities experiencing these events, realizing that explosions of hydrogen were considered in the design of the systems.
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Are there other ' accidental' or ' routine' _ releases of ;
radionuclides which the public is not aware of?"-
3 Answer:
The facilities' Technical Specifications require reporting all routine releases of radionuclides to the environment on a semi-annual basis. Significant levels of non-routine or j
accidental releases are also required to be reported to Q
the NRC within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> er 30 days deper. ding on the events.
These reports are in local public document rooms and are available to the public.
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Question: "6.
Who has the responsibility to notify the people of Minnesota when such accidents occur in Minnesota?"
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Answer:
The responsibility depends on the severity of the incident.
If it necessitates actuation of the plant's site emergency plan, the utility would notify appropriate state agencies in accordance with the plan.
l An offgas explosion involving release of radioactivity above NRC limits or requiring a. reactor shutdcup of several days would be the subject of an' NRC news release.
Lesser events would be covered by Licensee Event Reports and NRC inspec-tion reports which would be publicly available.
(Documents for Monticello are available at the Minneapolis Public Library.)
State officials would be notified if a press release were issued.
(Peter Vanderpoel, Chairman of the itinnesota Environ-j mental Quality Board has been designated by the Governor as the j
NRC Liaison Officer for the State.)
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